Abstract
If X is a geodesic metric space and x 1, x 2, x 3 ∈ X, a geodesic triangle T = {x 1, x 2, x 3} is the union of the three geodesics [x 1 x 2], [x 2 x 3] and [x 3 x 1] in X. The space X is δ-hyperbolic (in the Gromov sense) if any side of T is contained in a δ-neighborhood of the union of the two other sides, for every geodesic triangle T in X. If X is hyperbolic, we denote by δ(X) the sharp hyperbolicity constant of X, i.e. δ(X) = inf{δ ≥ 0: X is δ-hyperbolic}. In this paper we characterize the product graphs G 1 × G 2 which are hyperbolic, in terms of G 1 and G 2: the product graph G 1 × G 2 is hyperbolic if and only if G 1 is hyperbolic and G 2 is bounded or G 2 is hyperbolic and G 1 is bounded. We also prove some sharp relations between the hyperbolicity constant of G 1 × G 2, δ(G 1), δ(G 2) and the diameters of G 1 and G 2 (and we find families of graphs for which the inequalities are attained). Furthermore, we obtain the precise value of the hyperbolicity constant for many product graphs.
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Michel, J., Rodríguez, J.M., Sigarreta, J.M. et al. Gromov hyperbolicity in Cartesian product graphs. Proc Math Sci 120, 593–609 (2010). https://doi.org/10.1007/s12044-010-0048-6
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DOI: https://doi.org/10.1007/s12044-010-0048-6